Flexible ceramic member having a pre-loaded tensile force applying means

ABSTRACT

An elongated flexible member including a plurality of ceramic segments, each segment having at least two opposite surfaces that are flat and parallel. The segments are aligned in stacked relationship with their flat faces in abutting face-to-face relation and forced toward each other in the direction of their composite length with a force which is sufficient to maintain the segments in compression when subjected to conditions of thermal change and/or flexing of the member during use. A method for making the ceramic member is disclosed.

This application is a continuation-in-part of Ser. No. 273,308, filedJuly 19, 1972 now abandoned.

This invention relates to elongated flexible ceramic elements, and moreparticularly to an element of this type which is useful in applicationswhere the element is to be subjected to conditions of thermal changeand/or forces tending to bend the element along its length.

The physical properties and/or the chemical inertness of ceramicmaterials frequently suggest such materials for use in applicationswherein the material is to be fabricated into elongated articles such asa ceramic conduit for conveying corrosive chemicals. Not infrequently,such elongated elements are subjected to thermal change or forces, suchas vibration or frictional drag, which tend to bend or deflect theelement along its longitudinal axis.

Because of the relatively high cost and difficulty of manufacturingceramic elements in continuous lengths, for example lengths greater thanabout two feet, ceramic materials have heretofore been generally limitedto use in those situations where their relatively high cost is justifiedin order to obtain the advantages from the physical and/or chemicalproperties of the ceramic materials. Even in such special situationswhere ceramic lengths have been employed, it has been important toassure that the elongated elements neither bend nor are subjected tolocalized stresses, so as to avoid cracking and/or breaking of theelongated element. Consequently, the circumstances under which elongatedceramic elements could be used heretofore have been severely limited.

This invention also relates to systems in which there are at least twomembers, one of which is movable relative to the other and in frictionalengagement therewith. More particularly, the invention relates to such asystem in which at least one of the members is an elongated flexibleceramic member having a working or wear surface defining an area ofcontact between the members and which has improved physicalcharacteristics.

Examples of such systems include the combination of elongated foils incontact with a forming fabric in a Fourdrinier .[.of.]. .Iadd.or.Iaddend.other papermaking machine, a Uhle box which bears against aforming fabric or felt in a papermaking machine, and doctor blades foruse in contact with rotating drums or other moving members.

It is therefore an object of the present invention to provide anelongated flexible ceramic element. It is also an object of thisinvention to provide an elongated flexible ceramic element ofsubstantial length wherein the element comprises a plurality of ceramicsegments adapted to accommodate conditions of thermal change or bendingof the element within predetermined limits. Another object of thisinvention is to provide a method for the manufacture of an elongatedflexible ceramic element.

It is also an object of this invention to provide a system comprising atleast two members one of which is movable with respect to the other andin frictional engagement therewith and one of which is an elongatedflexible ceramic member.

Other objects and advantages of the invention will be recognized fromthe following description, including the drawings in which:

FIG. 1 is a representation of an elongated segmented ceramic memberembodying various features of the invention;

FIG. 2 is a sectional and fragmentary view of one end of the membershown in FIG. 1;

FIG. 3 is a representation of a segment of the member shown in FIG. 1;

FIG. 4 is a grossly exaggerated representation of a plurality ofsegments deflected in a manner to aid in explaining certain calculationsattending the disclosed invention;

FIG. 5 is a grossly exaggerated representation of a portion of adeflected composite of ceramic segments; and,

FIG. 6 is a representation of one embodiment of a system including atleast two relatively movable members and showing various features of theinvention.

One embodiment of a system which includes an elongated flexible ceramicmember and which includes at least two members, one of which is movablerelative to the other and in frictional engagement therewith is thedoctor system depicted in FIG. 6. This depicted system is a Yankee Dryer24 on which a paper web 20 is dried and creped. The web is trained abouta portion of the peripheral surface of the dryer 24 and dried by heattransferred through the cylindrical shell 22 thereof. Steam introducedinto the interior of the dryer shell is commonly used to heat the shell.The paper web 20 is doctored from the shell 22 by means of a doctorblade 26 as is well known in the art to provide a creped paper web 28.In this embodiment, the dryer shell 22 comprises a first member of thesystem and is movable relative to and in frictional engagement with thedoctor blade 26 which comprises a second member of the system.

In the system depicted in FIG. 6, the doctor blade 26 is positioned withrespect to the dryer surface 22 and to the paper web 20 by support meansshown generally at 25 including a pair of jaws 27 and 27' havingshoulders 29 and 29', respectively, that engage mating slots 31 and 31'in opposite surfaces of the doctor blade 26. Other suitable mountingmeans will be readily recognized by one skilled in the art.

In operation of the depicted system, the surface of the shell 22 becomesirregular due to its being heated by the steam. In order to keep thedoctor blade in contact with the shell for doctoring the web from theshell, it is necessary to bend the doctor blade so that it conforms tothe irregularities in the shell surface.

In this and other systems of this type, it is desired that one of themembers be flexible and have a good wear surface that is engaged by theother of the members. If has long been desired that such one of themembers be made of a ceramic material to take advantage of the wearresistance of this material. Continuous lengths of ceramic areprohibitively costly. Members having small ceramic inerts disposed alongthe length of the member to define a wear surface have been tried butsuch members develop gaps between the inserts where the member bendsduring use so that the edges and/or corners of the separated segmentsbecome points of excessive wear.

One of the members of the illustrated doctor blade embodiment comprisesan elongated flexible member 30 including a plurality of ceramicsegments 32 each having at least two opposite substantially parallelflat faces 34 and 36 (FIG. 3). (The member 30 comprises the doctor blade26 in FIG. 6). As illustrated, the ceramic segments are aligned inabutting face-to-face relation to define the composite 30, the faces 34and 36 being disposed substantially perpendicular to the longitudinalaxis, i.e. the composite length, of the member 30. The aligned segments32 are forced toward each other by a tension means 38 (FIG. 2) with aforce which will elastically compress the segments to the extent thatwhen the member is deflected during use the compression in thoseportions of the abutting segment faces disposed along the outside of theline of curvature of the deflected member is relieved to a degree lessthan that which will result in physical separation of such facepositions and the compression in those portions of the abutting segmentfaces disposed along the inside of the line of curvature of thedeflected member will increase to the degree necessary to accommodatethe deflection without physical destruction of such portions of thesegments.

Each of the segments 32 of the elongated member 30 of the presentdisclosure is fabricated from a hard dense ceramic material that isavailable at a reasonable cost.

The ceramic preferably is an impervious crystalline material thatcombines high mechanical strength with extreme hardness, inertness,refractoriness, and high chemical resistance properties. Because theseproperties are retained over a wide range of application andenvironmental conditions that many other materials cannot withstand,such ceramics suitably serve under conditions adverse to othermaterials. Alumina, silicon carbide, boron carbide and silicon nitridematerials possess those properties required in many industrialapplications, and are economically feasible for such end uses. Aluminais particularly suitable and is preferred for use in the present ceramicmember because of its properties and its availability at relatively lowcost when formed in relatively short segments.

The alumina segment 32 is formed by compacting finely ground oxidepowders with fluxing agents and inhibitors at relatively high pressuresas is known in the art. Forming methods include dry pressing, isostaticpressing, casting, extrusion, and injection molding. After forming, theresulting "green" ceramic segment is fired at a high temperature for aspecific length of time. Firing temperatures vary but usually rangebetween 2,500° F. and 3,250° F. During firing the ceramic shrinks;therefore, segments are formed while in the green state to allow for thephysical reduction caused by firing. After firing, the ceramic segmentis strong, hard and dense, composed substantially of pure alumina ofcontrolled crystal size. Machining of the ceramic segments is possibleeither before or after firing. Fired segments are ground or lapped toobtain the desired surfaces thereof. Grinding usually must be done withdiamond-impregnated wheels, although silicon-carbide or alumina wheelsare sometimes used.

Most of those physical properties desired in the ceramic segmentsimprove as the purity of the ceramic increases, especially hardness,compressive strength, wear resistance and chemical resistance. Forexample, alumina ceramic compositions having aluminum oxide contentsless than about 85% lose certain of their properties to an unacceptabledegree. Preferably, the alumina ceramics contain about 90.0% or morealuminum oxide.

The compressive strength of the ceramics exceed that for most materials.For example, compressive strengths as high as 550,000 psi have beenobtained in certain relatively pure alumina ceramics. Suitablecompressive strengths for the ceramic segments 32 range upwardly fromabout 200,000 psi.

Each of the segments 32 is provided with two opposite substantially flatand parallel faces 34 and 36. The segments are disposed in face-to-facerelation with their parallel faces abutting the parallel faces ofadjacent segments to define the elongated composite 30 of a desiredlength and subjected to a compressive force applied at substantiallyright angles to the faces. The flatness and parallelism of the abuttingsegment faces help to prevent cracking or breaking of the segments dueto unevenly applied stresses or localized stresses by distributing thecompressive forces evenly over the abutting faces. Abutting segmentfaces, each of which is flat to within about 0.0002 inches and has asurface finish of less than about 20 microinches arithmetic average (AA)have been found to be suitable for these purposes. When such individualsegments are placed in face-to-face relation without grout or adhesive,the abutting flat faces of adjacent segments lie in intimate contactwith each other over substantially the entire areas of the abuttingfaces without significant open space therebetween so that the abuttingfaces supply support to each other especially when the surface of themember is being ground as will be described hereinafter. In oneembodiment, each segment is provided with an opening 40 extendingbetween the opposite flat faces 34 and 36 thereof. This opening in asegment is aligned with similar openings of abutting segments to providea channel through the composite 30 for receiving a tension means 38 forcompressing the segments in the direction of their composite length.

As noted above, in producing an elongated member of given length, asufficient number of segments 32 are assembled in face-to-face relationwith their respective openings 40 aligned to obtain the desired length.The assembled segments are secured together with a force appliedsubstantially in the direction of the length of the composite 30 andsubstantially perpendicular to the flat parallel faces of the segments.This force is sufficient to place the segments in elastic compressionand is suitably applied as by a tension means 38 applying a compressiveforce to opposite ends 42 and 44 of the composite 30. One suitabletension means is a cable 38 inserted through the aligned openings 40extending between the opposite faces 34 and 36 of each of the assembledsegments, pulled to the required length, anchored at the opposite endsof the composite as by swage fittings 46 to exert a compressive forceupon the composite at its opposite ends. Alternatively, other tensioningmeans may be used to establish the desired compression of the segmentsin the composite. One such other means includes a rod disposed in thealigned openings 24 of the segments and fitted with a nut at one or bothof its ends so that tightening of the nuts tensions the rod and placesthe segments in compression. One suitable cable for applying the desiredcompression force to the segments is made of carbon steel and of thegeneral type employed in prestressed concrete structures.

The cable 38 may be chosen with a cross sectional area less than thecross sectional area of the opening 40 in each segment and after thesegment is in place on the cable the space between the cable and theinside surface of the opening in the segment may be filled with a grout48 (FIGS. 1 and 2), such as rigid polyurethane, to position the cablewithin the openings. One suitable grout is a liquid casting urethanepolymer designated as LD-2699, sold by E. I. Du Pont de Nemours Company,Trenton, N. J. This grout also accommodates the axial movement of thesegments with respect to the compression cable during compression of thecomposite and/or relative movement between the segments and the cable inthe event the member is subjected to thermal change during use.

As illustrated, the composite of segments is provided with a plate orother means such as a metallic segment 50 at each end of the compositeto provide for distribution of the compressive force over the face ofeach end segment to protect it from damage by localized forces. In thoseinstances where the desired compression is relatively great a pluralityof tension means, e.g. cables, provides greater compressive capability.In that event, the plurality of cables 38 are desirably threaded throughspaced apart, aligned openings through the segments. Such constructionaids in more evenly distributing the compressive forces over theabutting faces of the segments.

The flexibility of the member 30 is made possible by employingrelatively short segments (e.g. on the order of one inch long) heldtogether with a compressive force such that when the elongated composite30 deflects by a distance d, along its length (see FIG. 5), at least apart of the compression in those portions 52 and 54 of the abuttingfaces 56 and 58 of adjacent segments 32 disposed on the outside of theline of curvature A of the deflected composite 30 is relieved,permitting those portions of the segments to expand to accommodate thedeflection without physically separating. Importantly, the compressiveforce holding the segments together is less than the maximum compressivestrength of the ceramic material by an amount which permits thoseportions 60 and 62 of the abutting faces 56 and 58 of adjacent segmentson the inside of the line of curvature A of the deflected composite tobe compressed by an additional amount, causing these portions of thesegments to compress by an amount sufficient to accommodate thedeflection without destruction of the segments. In addition, the lengthof the individual segments is chosen to be sufficiently small as permitstheir manufacture at minimized costs taking into consideration theanticipated compressive forces to which the segments are to be subjectedin order to obtain the desired response of the composite incident todeflective forces.

In addition to the deflective forces, consideration must be given tothermal changes affecting the element in that such will usually producedifferent responses in the ceramic segments and the tension member. Suchthermal changes can arise by differences in the start-up and operatingtemperatures of the machine or system in which the element is installed,and/or changes in ambient temperature of the element during assembly,shipping or installation.

In calculating the compression required to accommodate the maximumanticipated deflection of a member of given length without separation ofthe segments, it is assumed that the deflection of the member will takethe shape of a uniformly loaded simple beam and that the maximumdeflection will be sufficiently small (less than about 1% of the memberlength) to permit the use of calculations based on circular arcs, ratherthan more exact curves. The latter could be used in those circumstanceswhere more exact calculations are required; however, it has been foundthat such is not necessary in constructing flexible members for most enduses. More specifically, and with reference to FIGS. 4 and 5, for amember 30 of given length, l (in inches), having a longitudinal axis,subjected to a maximum anticipated deflection, d (in inches), along aline of curvature A, and made up of a plurality of segments 32 eachbeing of a known .[.length, L (in inches), and a.]. dimension, h (ininches), across the segments, in the direction of the applied deflectiveforce and a cross-sectional area, A_(c), in square inches, thepreloading on the tension member, e.g. cable 38, which will impart tothe ceramic segments the necessary compressive force that precludesseparation of the segments is calcuated using the equation ##EQU1##where: E_(c) = the modulus of elasticity of the ceramic;

A_(c) = the cross-sectional area of a ceramic segment in a planeperpendicular to the composite length of the member, in square inches;

d = the maximum anticipated deflection of the member, in inches;

h = the dimension of a ceramic segment in the plane perpendicular to thecomposite length of the member and in alignment with the direction ofthe deflective force, in inches;

l = the overall length of the member. .[., and

L = the length of a ceramic segment, in inches.].

With reference to Equation (1), it is noted that the initiallydetermined preloading is divided by 2 to give the preloading to be usedin tensioning the cable 38. This fact arises because of the manner inwhich the ceramic segments are stressed when the member is deflectedwhile under compression. More specifically, assuming the cable 38 isdisposed midway between the ends of the segment dimension h, when themember is in an undeflected state, the stress on each compressed ceramicsegment is the same at any point along the dimension h. When the memberis deflected, the stress in that portion of a segment on the outside ofthe line of curvature (on the outside end of the dimension h) is reducedto zero and the stress in that portion of the same segment on the insideof the line of curvature is doubled. Thus when preloading the alignedsegments, the stress imparted to the segments is taken as the average ofthe stresses along the dimension h when the member is deflected by amaximum amount.

The effect of thermal change upon the member 30 must also be taken intoaccount. Thermal changes occur most frequently by reason of the member30 being manufactured at a first temperature, room temperature forexample, and thereafter encountering a substantially higher operatingtemperature. In such circumstances, the strain in the cable 38 decreaseswhen its temperature increases by reason of the cable expanding whenheated. Expansion of the cable cross-section as well as along its lengthis of importance. The ceramic also expands when heated, but usually to alesser extent than the cable, so that there is added to the preloadcalculated for deflection in accordance with Equation (1), an additionalpreloading which will compensate for the effect of thermal change uponthe cable and the ceramic and provide the desired preloading foraccommodating deflection up to a maximum temperature. Such additionalpreloading of the tension means is calculated using the equation##EQU2## where: α₃ = the coefficient of thermal expansion of the tensionmember;

α_(c) = the coefficient of thermal expansion of the ceramic;

ΔT = degrees of temperature change anticipated, in degrees F;

A_(s) = cross-sectional area of the tension member, in square inches;

E_(s) = the modulus of elasticity of the tension member;

A_(c) = the cross-sectional area of a ceramic segment in a planeperpendicular to the length of the member, in square inches;

E_(c) = the modulus of elasticity of the ceramic.

Combining Equations (1) and (2) gives ##EQU3## where P is the totalpreloading of the tension member which will prevent separation of thesegments of the member 30 when the member is deflected up to a maximumamount d while at a temperature less than an anticipated maximumtemperature. It will be noted that in those situations where the member30 will not experience a thermal change, ΔT will be zero and P_(T)[including its equivalent expression in Equation (3)] will be zero andno additional preloading will be required to account for thermalchanges.

Thus, in any given situation where the elongated member 30 is to besubjected to deflection forces, it is possible to select a compositewhich exhibits the desired non-separation of abutting segment faces whenthe composite is deflected along its composite length. As shown inEquation (1), the preloading force (compressive force) applied to thealigned segments, for any given maximum anticipated deflection and totallength of the segmented member, depends upon the length of eachindividual segment and the dimension h of each segment. Thus, if thedeflection capability of a given composite of ceramic segments is lessthan that which precludes physical separation of the abutting faces ofthe segments under the anticipated deflection, an adjustment can bemade, in many instances, in either the length or width of the individualsegments, or in both the length and width. Of course, consideration mustbe give to the added compression experienced by those portions of theabutting segment faces disposed on the inside of the line of curvatureof the deflected composite.

The preloading force exerted upon the ceramic segments is kept belowthat amount of force which will compress the ceramic material to withinabout one-half, and preferably to within about 20%, of its maximumcompressive strength to insure that localized stresses which may occurwithin the composite do not exceed such maximum compressive strengthwith resultant damage to one or more segments. This preferred preloadingalso provides a substantial margin of safety against damage to thesegments by inadvertent overloading of the segments to produce unduedeflection. In any event, the preloading of the segments is sufficientto shorten the length of each segment, hence shorten the overall lengthof the composite. Further, in the preferred preloading, the segments aresufficiently deformed at the interface between abutting segment faces asresults in substantial loss of joint identity at such interface. Suchdeformation is known to occur when the segments are preloaded to betweenabout 15% and 20% of the maximum compressive strength of the ceramic.This substantial loss of joint identity has been found to be importantin establishing the working surface on the member in that such allowsthe composite to be ground to a suitable smoothness. Less preloading isacceptable but at a loss of certainty of achieving the desiredproperties in the composite. Thus, the preloading of the ceramicsegments must be sufficient to maintain the segments abutting when themember is deflected by a maximum amount d but less than that preloadingwhich will compress the ceramic to more than one-half its totalcompressive strength.

It is understood that in the present discussion each of the segments issubstantially identical to each other segment in a given composite. Suchis assumed for purposes of simplifying the disclosure. It is notrequired, however, that all the segments be identical. For example, itmay be desirable to provide a segment member which is deflected bydifferent degrees along its length. In such an embodiment, thedeflective characteristics of the member will differ in differentportions of its length and the segments in each such portion may differin length from the segments in other portions of the length of themember.

As disclosed, one of the members of the system is movable with respectto the other member. In many embodiments, one member is held stationarywhile the other member moves thereover in frictional engagementtherewith. Similarly, in many embodiments the stationary member wll bethe flexible ceramic member 30 described above and will include aleading edge 72 which is initially contacted by the other member as itmoves over the ceramic member. In such instances it is important thatsuch leading edge be straight and free of irregularities such as gapsresulting from chipping of the leading edge inasmuch as suchirregularities, among other things, hinder or prevent alignment betweenthe two members and create wear points between the moving members.

The segmented member 30, being intended for use in a system where it isin frictional engagement with a further member and there is relativemovement between the members, is provided with an elongated smoothworking surface 70. This surface 70 extends along the length of themember 30 and defines an extended area of contact between the relativelymoving members. Minimum wear of this surface and of the other of themoving members is obtained by maximizing the smoothness of this workingsurface. This is accomplished by grinding the surface 70 after thesegments have been formed into the composite 30 and preloaded asdescribed hereinabove.

In a typical grinding operation the segmented member 30 is anchored onthe bed of a grinding machine. A diamond impregnated grinding wheel,preferably of the type having an annular planar grinding surface is usedin the grinding process. This grinding wheel is moved into contact withthe segmented member with the plane of the grinding surface of thegrinding wheel disposed at a slight angle with respect to the plane ofthe surface to be ground so that only a portion of the rotating grindingsurface is in contact with the segments at any given time. Preferablythe grinding surface plane is also disposed with respect to the workingsurface so that grinding of the surface takes place as the annulargrinding surface moves onto the surface and little or no grinding takesplace as the grinding surface is moving away from the surface beingground.

The rotation of the grinding wheel, when grinding a leading edge of thetype shown in FIG. 1, is such that the grinding surface initiallycontacts the leading edge 72 as the grinding surface moves toward thatedge. In this manner, the grinding forces exerted upon the segments aredirected inwardly of the segments to aid in preventing chipping of thesegments edges during grinding. Preferably, the grinding action at theleading edge is in a direction substantially perpendicular to theleading edge. Variations of greater than about 10 degrees from suchperpendicular relationship provide relatively poor edges.

In the grinding operation the compression of the segments in thedirection of their composite length maintains the edges of abuttingsegments in supporting relationship to each other. In addition to thisphysical support of one segment by its neighbor, the compression in thesegments is sufficient to prevent the force of the grinding operationfrom placing the segment edges in tension as the grinding wheel dragsacross the segment, thereby enhancing the resistance of the segments toedge chipping during grinding. This results in an improved smoothness ofthe working surface 70 and is believed to be responsible in part for thegood surface finishes obtained by applicant when grinding the segmentedmember as disclosed herein.

The grinding operation disclosed herein provides surface finishes of theworking surface of less than about 20 microinches (AA). This degree ofsurface smoothness has been found to impart exceptionally good wearcharacteristics to both moving members of the system. Moreover, edgesdeveloped by such grinding procedures are substantially straight linesand substantially free of chipped out portions. Such leading edge alsohas a smoothness substantially equivalent to the smoothness of theremainder of the working surface 70.

EXAMPLE I

A doctor blade for doctoring a paper web from the surface of acylindrical dryer shell is made as follows. Such doctor blades normallyare deflected by different amounts along different portions of theirlength due to undulations in the dryer shell across its width. The mostsevere deflection is chosen and the total deflection capability of theblade is made sufficient to accommodate it. In this Example the length,l, of the chosen deflected portion is 50 inches.

The doctor blade in the configuration illustrated in FIG. 1 is made fromone inch long (L) alumina segments (AD-995 from Coors Porcelain Co.)each having a cross-sectional area (A_(c)) of 0.78 square inches. Thedimension .[.(H).]. .Iadd.(h).Iaddend., the dimension in the directionof the application of the deflective forces, is 0.875 inch. Thesesegments are aligned with their flat parallel faces abutting andcompressed in the direction of their composite length by a stainlesssteel cable of 0.14 square inches cross-sectional area threaded throughaligned openings in the segments.

The maximum anticipated deflection of the doctor blade over the chosen50 inch length, l, is determined to be 0.027 inch and the anticipatedthermal change is from 70° F. to 300° F. (ΔT = 230° F). The preloadingfor the cable which passes through the segments is calculated usingEquation (3) as follows: ##EQU4## P = 1579.5 + 2385.19 P = 3964.69pounds

This preloading imparted a compressive force to the ceramic which isabout 1.54% of the 330,000 psi approximate maximum compressive strengthfor AD-995 alumina. This degree of compression provides for theanticipated deflection, occurring at a temperature of 300° F., withoutcomplete relief of the compression in those portions of the abuttingsegment faces furtherest from the longitudinal axis of the member alongwhich the deflection occurs and, importantly, provides for additionalcompression of those portions of the abutting segment faces nearest thelongitudinal axis of the member as necessary to accommodate thedeflection.

The working surface 70 of the segmented member 30 is ground while themember is supported along its entire length on the bed of a grindingmachine. A five hundred grit diamond impregnated wheel, having anannular grinding surface, as sold by the Norton Company is employed inthe grinding operation. The grinding wheel has a diameter of 10 inches,and is rotated at approximately 3600 revolutions per minute. The wheelis moved along the length of the working surface at a speed betweenabout 10 and 20 feet per minute. The position of the grinding wheelrelative to the working surface and its rotational movement is asdescribed above. The grinding operation provides a surface finish ofabout 20 microinches (AA) with no significant chipping of the leadingedge 72.

EXAMPLE II

Another system of the type disclosed herein comprises a foil and aforming fabric of a Fourdrinier papermaking machine. In this system, theelongated foil is disposed beneath the forming fabric and serves tosupport the fabric and remove water from a slurry of papermaking fiberscarried on the fabric. In these functions, the fabric slides over thefoil while it is pulled against the foil by suction developed by thefoil. There is substantial wear of both the foil and the wire in thesesystems as known heretofore.

A 200 inch long foil for use in a Fourdrinier papermaking machine ismade from 200 one inch long AD-995 alumina segments held in compressionby a 0.677 inch diameter stainless steel cable which is passed throughan opening located centrally of each segment. Each segment has across-sectional area (A_(c)) of 2 square inches, and a dimension (h) of2 inches. The maximum anticipated deflection of the foil is 0.5 inch andthe anticipated thermal change is from 70° F. to 170° F. (ΔT = 100° F).

Using Equation (3), the preloading for the cable for preventingseparation of the segments under such conditions is calculated asfollows: ##EQU5## P = 10,746 + 3,557.8 P = 14,304 pounds

The preload force in this example stresses the ceramic to 2.17% of itsmaximum compressive strength.

This foil is provided with a ground elongated working surface having asmoothness of less than about 20 microinches AA in the manner disclosedherein. In use, the foil exhibits excellent wear qualities and does notexhibit gaps between abutting segment faces. Foils of this type whenused in a high speed Fourdrinier papermaking machine do not producestreaks in the paper web formed on the forming fabric moving over thefoil, as has been experienced by the prior art segmented foils whichdevelop gaps between abutting segments.

EXAMPLE III

A further system of the type disclosed herein comprises a suction devicefor use in a papermaking machine known as a Uhle Box. This suctiondevice comprises an elongated trough-like device having an elongatedslot extending along its length and opening toward a forming fabric orfelt moving thereacross. A suction is developed within the Uhle Box sothat the fabric or felt is pulled against the edges of the slot andwater or other material is pulled from the fabric or felt into the UhleBox. The edges of the slot are subjected to relatively great wear forcesand the Uhle Box, hence the slot edges, are subjected to substantialdeflective forces as the fabric or felt moves across the device in adirection transverse to its length.

A Uhle Box having each of its slot edges made of a flexible ceramicmember may be fabricated using the teachings of the invention asfollows. Each such slot edge is 200 inches long and made of 1 inch longAD-995 alumina segments held in compression by a stainless steel cablehaving a cross sectional area of 0.25 square inches which is disposed inaligned openings in the segments. Each segment has a cross-sectionalarea (A_(c)) of 0.92 square inch and a dimension (h) of 1.250 inches.

In calculating the preload for the cable, the maximum anticipateddeflection is 3 inches and the maximum temperature anticipated duringuse is 170° F. The temperature at assembly is 70° F, giving a ΔT of 100°F. Using Equation (3) the preload is determined as follows: ##EQU6## P =18,605 + 1,771.5 P = 20,376.5 pounds

The preload force in this Example stresses the ceramic ot 6.71% of itsmaximum compressive strength.

That portion of the ceramic member which engages the moving fabric orfelt is ground to a surface smoothness of less than 20 microinches AA bythe procedures set forth above. As in Examples I and II, the ceramicmember exhibits good wear charcteristics and does not develop gapsbetween the segment faces when deflected by the anticipated maximumamount.

While preferred embodiments have been shown and described, it will beunderstood that there is no intent to limit the invention by suchdisclosure but rather, it is intended to cover all modifications andalternative constructions falling within the spirit and scope of theinvention as defined in the appended claims.

What is claimed is:
 1. In a system including at least two members one ofwhich is movable relative to the other and in frictional engagementtherewith, such as members in a papermaking system, the improvementwherein the other member is an elongated flexible ceramic elementcomprisinga plurality of ceramic segments, each having at least twoopposite surfaces that are substantially flat and parallel, saidsegments being aligned with their flat faces in abutting face-to-facerelation and in respective planes that are oriented substantiallyperpendicular to the composite length of said plurality of segments,tension means anchored to opposite ends of said element and forcing saidsegments toward each other in a direction along their composite lengthand substantially perpendicular to their respective parallel faces witha preload force on said tension means, when said latter member is in anundeflected condition, that is at least the force calculated by theequation: ##EQU7## where: P is the preload of said tension means, inpounds; E_(c) is the modulus of elasticity of the ceramic material;A_(c) is the cross-sectional area of a ceramic segment in a planeperpendicular to the composite length of said elongated ceramic elementin square inches; d is the maximum anticipated deflection of saidelongated ceramic element, in inches; h is the dimension of a ceramicsegment in the plane perpendicular to the composite length of saidelongated ceramic element and in alignment with the direction of saiddeflective force, in inches; l is the overall length of said lattermember; α_(s) is the coefficient of thermal expansion of said tensionmeans; α_(c) is the coefficient of thermal expansion of said ceramic;Δ_(T) is the degree of temperature change anticipated, in degrees F.;A_(s) is the cross-sectional area of said tension means, E_(s) is themodulus of elasticity of said tension means, .[.and L is the length of aceramic segment, in inches,.].but less than the amount of preload forcewhich will compress said ceramic to over about one-half of its maximumcompressive strength, whereby loading forces exerted upon said elongatedceramic element are directed thereagainst in a direction substantiallyperpendicular to the longitudinal dimension thereof and deflection ofsaid elongated ceramic element pursuant to such loading forces iscompensated for in said compressed segments by further compression ofsaid segments in those portions of the abutting faces thereof disposedalong the inside of the line of curvature of said elongated ceramicelement and by relief of less than all of the compression in thoseportions of said abutting faces that are disposed along the outside ofsaid line of curvature of said elongated ceramic element withoutphysical separation of said segments at their abutting faces.
 2. Anelongated ceramic element in accordance with claim 1 wherein saidpreload force does not exceed that preload force which will compresssaid segments to greater than about 20% of their maximum compressivestrength.
 3. An elongated ceramic element in accordance with claim 1wherein said tension means forcing said segments toward each other is anonceramic material.
 4. An elongated ceramic element in accordance withclaim 1 wherein said ceramic comprises alumina.
 5. An elongated ceramicelement in accordance with claim 4 wherein said alumina has a purity ofgreater than about 85%.
 6. An elongated ceramic element in accordancewith claim 1 wherein said ceramic segments are substantially identicalto one another and each has an opening extending between its oppositeflat and parallel surfaces, said openings in said segments being inregister and said tension means extending therethrough.
 7. An elongatedceramic element in accordance with claim 1 wherein each of the abuttingflat faces of said ceramic segments is flat to within about 0.0002inches.